A multicarrier transmission system is one that employs Frequency Division Multiplexed (FDM) subcarriers for transmission of data across a communication channel.
A typical multicarrier transmission system is depicted in FIG. 1. It shows serial data in the form of bits input to a serial to parallel converter 100, which also frames the bits. (A frame is an ordered sequence of bits of a given size). The output bit frame is mapped to symbols in the mapper 300. The frame of symbols then passes through the modulator 400. The modulation could be carried out, for instance, using Inverse Fast Fourier Transform (IFFT) and the corresponding demodulation carried out using Fast Fourier Transform (FFT). The modulated signal is converted to serial data (block 700) and transmitted on to the channel 900, where it suffers several impairments, in addition to being corrupted by noise. At the receiver, the received data is grouped into frames by a serial to parallel converter 1000 and subsequently passed through the demodulator 1100. The demodulated symbols are converted to bits through an inverse symbol mapper 1300, which also performs parallel to serial conversion.
The data bits input to the transmitter module are independent and the symbols obtained by the bits-to-symbol mapper are also independent. Whenever the number of subchannels is large, the modulator output magnitude tends to have a Gaussian distribution, which has an infinite peak to average power ratio. A clipper 800, which has a cutoff based on a predesignated clipping probability, is used to limit the peak signal power that is transmitted on to the channel. The clipping probability is kept low so that the error introduced due to clipping is small. But if the predesignated clipping probability is reduced, the increase in peak power may cause saturation in the subsequent amplification stages and intermodulation distortion due to nonlinearities in the transmission medium. Thus it becomes necessary to limit the peak power transmitted on to the channel by reducing the clipping level while still maintaining a low clipping probability.
FIG. 1 shows a peak detector 500 at the output of the modulator, which activates a symbol modifier 600 whenever the magnitude of any of the modulator output samples exceeds a predesignated threshold or set of thresholds. Although FIG. 1 shows the symbol modifier correcting the symbols that are being modulated, it is possible to achieve the same effect by correcting the modulated samples. The threshold could be set based on the level of peak power that can be transmitted on to the channel, while the task of the symbol modifier is to modify the symbols being modulated in such a manner that the peak is reduced. The symbol modifier needs to take care of a number of objectives:                1. The clipping probability should remain unaltered when the clipping threshold is lowered.        2. The loss in data rate caused due to symbol modification should be minimal.        3. A preferable requirement is that the symbol modifier should have a low complexity.        4. The PAR reduction scheme should be transparent to the receiver.        5. The modifications made to the data symbols should not cause an increase in the average subchannel powers.        6. The modifications introduced in the symbols should not cause significant increase in the error rate at the receiver end.        
Most of the above requirements are conflicting and it may not be possible to meet all of them at the same time. The amount of importance given to each of these requirements by a particular method decides the amount of Peak to Average power Ratio (PAR) reduction achievable by that method.
Most of the prior art addresses several of the requirements listed above, but not all of them together. Some of the conventional methods use Peak Reduction Tones (PRT) (a predesignated set of subchannels) to reduce the PAR. One of the PRT methods is described by J. Tellado and J. Cioffi in “PAR Reduction in Multicarrier Transmission Systems (97-367)”, T1E1.4/97-367 Dec. 8, 1997. It uses an iterative procedure which aims at reducing the largest peak every iteration. Another method is described by the same authors in “Revisiting DMT's PAR (98-083)”, T1E1.4/98-083, Mar. 3, 1998. It uses an iterative procedure that tries to reduce all the peaks present during each iteration. Both the methods employ a precomputed peak reduction kernel. The reduction in PAR is obtained at the loss of several data carrying subchannels. PRT methods require that the receiver be aware of the predesignated subchannels to be used for peak reduction.
The modulo-D scheme described in “A new approach to PAR control in DMT Systems”, NF-83, ITU-T, Study Group 15, 11-14 May 1998 uses an expanded constellation in each subchannel for PAR reduction. The method uses symbols drawn from larger constellations whenever the original modulation produces large peaks. The use of larger constellations results in increased average power and higher intermodulation distortion. The receiver will have to perform a reverse operation to the transmitter action to recover the original data symbols.
A sign inversion method has been specified in “An Efficient Implementation of PAR reduction method based on subset inversion”, AB-061r2, ITU-T, Study Group 15, 3-7 August, 1998. It consists of dividing the set of subchannels into several subsets and achieving a PAR reduction by inverting the sign of the symbols in one or more of the subsets. The transmitter uses part of the data carrying bits to indicate to the receiver whether a sign inversion has been applied. This method requires receiver cooperation and results in data rate loss. Additionally, any error in a subset inversion indicator bit will result in incorrect decoding of data bits associated with that subset.